The invention relates to a transfer unit for an electrophotographic copying machine, and more particularly, to a transfer unit which transfers a toner image, formed by developing an electrostatic latent image developed by an exposure step, onto a transfer sheet.
Referring to FIG. 1, there is shown a conventional electrophotographic copying machine which is shown to define an original feed path to permit a copying from a single sheet-shaped original 1. The original is placed on an inclined original guide table 2, and is fed in a direction indicated by an arrow A into an inlet opening of an original feeder comprising pairs of conveying rollers 4A, 4B, 5A, 5B and guide plates 6, 7. The pair of vertically aligned conveying rollers 4A, 4B feeds the original 1 toward an exposure window 8 which is formed in the lower guide plate 7. During such movement, the original 1 passes between the both guide plates 6, 7. After passage through the exposure window 8, the original 1 is further conveyed by another pair of vertically aligned conveying rollers 5A, 5B to be delivered onto an original tray 9.
As the original 1 is fed by the original feeder, a pair of microswitches 3A, 3B located on the opposite sides of the conveying roller 4A detect the position of the original 1 to provide an output, which is utilized to control the timing of operation of various parts of the electrophotographic copying machine. As the original 1 passes through the exposure window 8, an illumination lamp 10 illuminates the surface of the original, whereby the image of the original is projected by an optics 11 onto a photosensitive drum 12. The drum 12 is adapted to rotate in a direction indicated by an arrow B, and is uniformly charged by a corona charger 14 after any residual charge has been removed by a neutralizer lamp 13. Subsequently, the drum surface is subject to an irradiation with the optical image of the original, thereby forming an electrostatic latent image thereon. The latent image is then developed with a toner by a developing unit 15 of dry type, and as the drum 12 continues to rotate, the toner image is carried into a transfer station 16.
A number of transfer sheets 18 are stored in a stack in a cassette 17 and are fed one by one by an oscillating and rotating feed roller 19, and a pair of vertically aligned register rollers 20 controls the timing to feed the sheet into the transfer station 16. In the transfer station 16, the transfer sheet 18 is fed into the nip between the drum 12 and transfer member 21 or a transfer roller to which a bias voltage is applied so as to be brought into overlapping relationship with the toner image on the drum, thus effecting a transfer of the toner image. Since the transfer sheet is conveyed in close contact with the drum 12 during such process, it is separated from the drum by the combined action of a separating claw 22 and an airstream to be described later. After the transfer step, the transfer sheet having the toner image transferred thereto is conveyed by a pair of vertically aligned conveying rollers 24 to move along a guide 23 and through a heat fixing unit 25 which includes heaters where the toner image is fixed by melting. Subsequently, the sheet is conveyed by another pair of vertically aligned delivery rollers 26 onto a copy tray 27.
Any residual toner which remains on the drum 12 without being transferred to the transfer sheet is swept off by a rotating cleaning brush 28 and is withdrawn by an airstream created by a fan 29 to be trapped by a filter 30. The cleaning brush 28, the fan 29 and the filter 30 are enclosed within a casing 31 in order to obtain an effective displacement of the residual toner and to prevent a dispersion of the toner into the machine. The airstream discharged by the fan 29 is introduced into a duct 32 having its outlet 32a located adjacent to the transfer station 16 so as to cooperate with the separating claw 22 to separate effectively the transfer sheet from the drum 12.
During the transfer step of the copying machine described above, the transfer sheet 18 is positively charged by the transfer roller 21 to which a bias voltage E is applied, as shown in FIG. 2, whereby the toner having a negative charge and deposited on the latent image formed on the drum 12 is attracted and migrates to the surface of the transfer sheet 18 as it is held between the drum 12 and the transfer roller 21, thus transferring the toner image onto the transfer sheet.
However, it is well recognized that the transfer quality is greatly degraded under high humidity conditions since the transfer sheet absorbs humidity to have its resistivity reduced. As the resistivity of the transfer sheet is reduced, the charge migrates in a direction of the plane of the transfer sheet 18, or in a direction perpendicular to the thickness thereof, as shown in FIG. 3, whereby it may leak through a metal roller 20 or the like which is used to guide the path of the transfer sheet, resulting in a failure to develop a transfer field.
As an approach to avoid such difficulty, there is proposed the isolation from the ground of a conveying path which is contacted by the transfer sheet during the time the transfer bias is applied to the transfer sheet, thereby preventing a leakage of the charge. However, if the transfer sheet used has an increased size, the isolation must cover an increased number of parts, resulting in an expensive arrangement.
Another approach has been proposed as shown in FIG. 4 where a pair of guide plates 40 are disposed close to the transfer roller 21 so as to delineate the upper and lower limit of the conveying path for the transfer sheet 18. An auxiliary bias voltage E.sub.0 of the same polarity as the bias voltage E applied to the transfer roller 21 is applied to the both guide plates 40 in order to avoid a leakage of the transfer charge therethrough. FIG. 5 shows an alternative approach in which the pair of guide plates 40 is replaced by a pair of conveying rollers 41 to which the bias voltage E.sub.0 is again applied.
However, the arrangement of FIG. 4 suffers from the disadvantage that the transfer sheet 18 passing between the pair of guide plates 40 may not contact the latter in a reliable manner. In the arrangement of FIG. 5, the contact between the rollers 41 and the transfer sheet 18 is assured, but it is necessary to provide a drive mechanism for these rollers, resulting in a complex arrangement.